Preventing evaporator coil freeze during re-heat dehumidification

ABSTRACT

In an embodiment, a method of preventing evaporator coil freeze in a heating, ventilation and air conditioning (HVAC) system includes determining a reference saturated suction temperate (SST) via a sensor disposed in relation to an evaporator coil in the HVAC system, where the HVAC; system is operating in reheat dehumidification mode. The method also includes determining whether the reference SST is below a minimum SST threshold. The method also includes, responsive to a determination that the reference SST is below the minimum SST threshold, determining a decreased compressor speed. The method also includes modulating a variable-speed. compressor in the HVAC system in correspondence to the decreased compressor speed.

TECHNICAL FIELD

The present disclosure relates generally to heating, ventilation, andair conditioning (HVAC) systems and more particularly, but not by way oflimitation, to control systems and methods for preventing evaporatorcoil freeze during re-heat dehumidification.

BACKGROUND

HVAC systems are used to regulate environmental conditions within anenclosed space. Typically, HVAC systems have a circulation fan thatpulls air from the enclosed space through ducts and pushes the air backinto the enclosed space through additional ducts after conditioning theair (e.g., heating, cooling, humidifying, or dehumidifying the air). Todirect operation of the circulation fan and other components, HVACsystems include a controller. In addition to directing operation of theHVAC system, the controller may be used to monitor various components,(i.e. equipment) of the HVAC system to determine if the components arefunctioning properly.

SUMMARY

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform the actions. One or morecomputer programs can be configured to perform particular operations oractions by virtue of including instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the actions.

In an embodiment, one general aspect includes a method of preventingevaporator coil freeze in a heating, ventilation and air conditioning(HVAC) system. The method includes determining a reference saturatedsuction temperate (SST) via a sensor disposed in relation to anevaporator coil in the HVAC system, where the HVAC system is operatingin reheat dehumidification mode. The method also includes determiningwhether the reference SST is below a minimum SST threshold. The methodalso includes, responsive to a determination that the reference SST isbelow the minimum SST threshold, determining a decreased compressorspeed. The method also includes modulating a variable-speed compressorin the HVAC system in correspondence to the decreased compressor speed.Other embodiments of this aspect include corresponding computer systems,apparatus, and computer programs recorded on one or more computerstorage devices, each configured to perform the actions of the method.

In an embodiment, another general aspect includes a heating, ventilationand air conditioning (HVAC) system. The HVAC system includes anevaporator coil, a re-heat coil, a condenser coil and a sensor disposedin relation to the evaporator coil. The HVAC system also includes acompressor fluidly coupled to the condenser coil, the evaporator coiland the re-heat coil. The HVAC system also includes a controlleroperatively coupled to the compressor, where the controller is operableto perform a method. The method includes determining a referencesaturated suction temperate (SST) via a sensor disposed in relation toan evaporator coil in the HVAC system, where the HVAC system isoperating in reheat dehumidification mode. The method also includesdetermining whether the reference SST is below a minimum SST threshold.The method also includes, responsive to a determination that thereference SST is below the minimum SST threshold, determining adecreased compressor speed. The method also includes modulating avariable-speed compressor in the HVAC system in correspondence to thedecreased compressor speed.

In an embodiment, another general aspect includes a computer-programproduct that further includes a non-transitory computer-usable mediumhaving computer-readable program code embodied therein. Thecomputer-readable program code is adapted to be executed to implement amethod. The method includes determining a reference saturated suctiontemperate (SST) via a sensor disposed in relation to an evaporator coilin the HVAC system, where the HVAC system is operating in reheatdehumidification mode. The method also includes determining whether thereference SST is below a minimum SST threshold. The method alsoincludes, responsive to a determination that the reference SST is belowthe minimum SST threshold, determining a decreased compressor speed. Themethod also includes modulating a variable-speed compressor in the HVACsystem in correspondence to the decreased compressor speed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference may now be had to thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a block diagram of an illustrative HVAC system;

FIG. 2A is a schematic diagram of a package HVAC system having a re-heatloop;

FIG. 2B is a schematic diagram of a split HVAC system having a re-heatloop;

FIG. 3 illustrates an example implementation involving multiplecompressors;

FIG. 4 is a flow diagram of a process for configurably modulatingcompressor speed during re-heat dehumidification mode;

FIG. 5 is a graph that shows example variation in modified compressorspeeds; and

FIG. 6 is a graph that illustrates example compressor-speed modulationfor an example multi-compressor HVAC system.

DETAILED DESCRIPTION

Various embodiments of the present disclosure will now be described morefully with reference to the accompanying drawings. The invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein.

HVAC systems are frequently utilized to adjust both temperature ofconditioned air as well as relative humidity of the conditioned air. Acooling capacity of an HVAC system is a combination of the HVAC system'ssensible cooling capacity and latent cooling capacity. Sensible coolingcapacity refers to an ability of the HVAC system to remove sensible heatfrom conditioned air. Latent cooling capacity refers to an ability ofthe HVAC system to remove latent heat from conditioned air. In a typicalembodiment, sensible cooling capacity and latent cooling capacity varywith environmental conditions. Sensible heat refers to heat that, whenadded to or removed from the conditioned air, results in a temperaturechange of the conditioned air, Latent heat refers to heat that, whenadded to or removed from the conditioned air, results in a phase changeof, for example, water within the conditioned air. Sensible-to-totalratio (“S/T ratio”) is a ratio of sensible heat to total heat (sensibleheat+latent heat). The lower the S/T ratio, the higher the latentcooling capacity of the HVAC system for given environmental conditions.

Sensible cooling load refers to an amount of heat that must be removedfrom the enclosed space to accomplish a desired temperature change ofthe air within the enclosed space. The sensible cooling load isreflected by a temperature within the enclosed space as read, forexample, on a dry-bulb thermometer. Latent cooling load refers to anamount of heat that must be removed from the enclosed space toaccomplish a desired change in humidity of the air within the enclosedspace. The latent cooling load is reflected by a temperature within theenclosed space as read, for example, on a wet-bulb thermometer. Setpointor temperature setpoint refers to a target temperature setting of theHVAC system as set by a user or automatically based on a pre-definedschedule. Discharge air temperature (DAT) refers to a temperature of airleaving an evaporator coil. Typically, DAT is maintained at a constantpre-set level. DAT varies with indoor dry-bulb air temperature, indoorwet-bulb air temperature, indoor air flow rate, cooling capacity of theHVAC system, and other design parameters.

When there is a high sensible cooling load such as, for example, whenoutside-air temperature is significantly warmer than an inside-airtemperature setpoint, the HVAC system will continue to operate in aneffort to effectively cool and dehumidify the conditioned air. Suchoperation is commonly referred to as “cooling mode.” When there is a lowsensible cooling load but high relative humidity such as, for example,when the outside air temperature is relatively close to the inside airtemperature setpoint, but the outside air is considerably more humidthan the inside air, additional steps must be undertaken to increase themoisture-removal capability of the HVAC system to avoid occupantdiscomfort.

One approach to air dehumidification involves lowering the temperaturesetpoint of the HVAC system. This approach causes the HVAC system tooperate for longer periods of time than if the temperature setpoint ofthe HVAC system were set to a higher temperature. This approach servesto reduce both the temperature and humidity of the conditioned air.However, this approach results in over-cooling of the conditioned air,which over-cooling often results in occupant discomfort.

Another air dehumidification approach involves re-heating of air leavingan evaporator coil. This approach typically involves directingrefrigerant from the compressor to a re-heat coil positioned adjacent tothe evaporator coil. The re-heat coil transfers some heat energy fromthe refrigerant to the air leaving the evaporator thereby raising thetemperature of air leaving the evaporator and lowering the temperatureof the refrigerant before the refrigerant moves to the condenser. Suchoperation is commonly referred to as “re-heat dehumidification mode.”

While re-heat dehumidification mode can improve occupant comfort ascompared to lowering the temperate setpoint, it presents challenges withrespect to saturation suction temperature. Saturation suctiontemperature (SST) refers to saturated refrigerant temperature at suctionpressure leaving an evaporator, or any measurement used as a proxy forsuch temperature. If the SST approaches a freezing point of therefrigerant, frost will begin to form on the evaporator coil. Thissituation is often referred to as evaporator coil freeze. Evaporatorcoil freeze causes an increased risk of damage to the evaporator coiland other components of the HVAC system. This problem can beparticularly common in HVAC systems that include more than compressor,as the presence of multiple circuits in the evaporator coil, some ofwhich are more downstream than others, can result in wide variation inSST throughout the evaporator coil.

The present disclosure recognizes that SST is not necessarily impactedthe same way in re-heat dehumidification mode as it is in cooling mode.Table 1 below illustrates three example scenarios for an example HVACsystem. In the first example scenario, the example HVAC system is incooling mode with a refrigerant charge of 16 pounds. In the secondexample scenario, the example HVAC system is in re-heat dehumidificationmode with the same refrigerant charge of 16 pounds. In the third examplescenario, the example HVAC system is again in re-heat dehumidificationmode but with a refrigerant charge of 20 pounds. For illustrativepurposes, the refrigerant is assumed to be R-410A, although any numberof other refrigerants could be substituted.

With reference to the first example scenario shown in Table 1, theexample HVAC system performs appropriately in cooling mode with therefrigerant charge of 16 pounds. However, with reference to the secondexample scenario, the example HVAC system performs comparatively poorlyin re-heat dehumidification mode with the same refrigerant charge of 16pounds. More specifically, in the second example scenario, the exampleHVAC system exhibits lower latent capacity, higher superheat (i.e., 41°F. of superheat), and lower suction pressure. This lower suctionpressure corresponds to an SST of approximately 34° F. and thus presentsincreased risk of evaporator coil freeze. However, in the thirdscenario, the example HVAC system performs appropriately in re-heatdehumidification mode with an additional four pounds of refrigerant.

TABLE 1 Example Scenario 1: Cooling Mode with Refrigerant Charge of 16pounds Example Scenario 2: Re-heat dehumidification mode withRefrigerant Charge of 16 pounds Example Scenario 3: Cooling Mode withRefrigerant Charge of 20 pounds Refrigerant Charge (LBS) 16 16 20Standard CFM 1,750 1,751 1,750 Suction Pressure (PSIG) 139 106 137 TotalCapacity (BTU/HR) 60,000 14,453 24,325 Sensible Capacity (BTU/HR) 41,0002,516 5,086 Latent Capacity (BTU/HR) 19,000 11,937 19,239 S/T Ratio 0.680.17 0.21 Subcooling (° F.) 7 0 2 Superheat (° F.) 11 41 15

The present disclosure recognizes that HVAC systems are oftenundercharged in accordance with the first and second example scenariosdiscussed above relative to Table 1, such that performance is acceptablein cooling mode but unacceptable in re-heat dehumidification mode. Duethis factor and other factors, such as SST variance throughoutmulti-compressor systems, re-heat dehumidification mode can present asignificant risk of evaporator coil freeze. Furthermore, since re-heatdehumidification mode operates by re-heating air leaving the evaporatorcoil, control methods based on DAT are typically ineffective when HVACsystems are operating in this mode.

The present disclosure describes examples of reducing a risk ofevaporator coil freeze using control systems and methods that areeffective, for example, in re-heat dehumidification mode. In variousembodiments, a controller of an HVAC system monitors SST and causes acompressor speed of a variable-speed compressor to be algorithmicallyadjusted, or modulated, based, at least in part, on the SST. Forexample, the compressor speed can be algorithmically decreased as theSST falls beneath a configurable minimum temperature, or whenever theSST indicates a trend towards evaporator coil freeze. Afterwards, thecompressor speed can be algorithmically increased towards its previouslevel as the SST moves toward or exceeds the configurable minimumtemperature, or whenever the SST indicates a trend away from evaporatorcoil freeze. Examples will be described below with reference to theDrawings.

FIG. 1 illustrates an HVAC system 100. In a typical embodiment, the HVACsystem 100 is a networked HVAC system that is configured to conditionair via, for example, heating, cooling, humidifying, or dehumidifyingair within an enclosed space 101. In a typical embodiment, the enclosedspace 101 is, for example, a house, an office building, a warehouse, andthe like. Thus, the HVAC system 100 can be a residential system or acommercial system such as, for example, a roof top system. Forillustration, the HVAC system 100 as illustrated in FIG. 1 includesvarious components; however, in other embodiments, the HVAC system 100may include additional components that are not it but typically includedwithin HVAC systems.

The HVAC system 100 includes a variable-speed circulation fan 110, are-heat coil 120 associated with the variable-speed circulation fan 110,typically, and a refrigerant evaporator coil 130, also typicallyassociated with the variable-speed circulation fan 110. Thevariable-speed circulation fan 110, the re-heat coil 120, and therefrigerant evaporator coil 130 are collectively referred to as an“indoor unit” 148. In a typical embodiment, the indoor unit 148 islocated within, or in close proximity to, the enclosed space 101. TheHVAC system 100 also includes a variable-speed compressor 140 and anassociated condenser coil 142, which are typically referred to as an“outdoor unit” 144. In various embodiments, the outdoor unit 144 is, forexample, a rooftop unit or a ground-level unit. The variable-speedcompressor 140 and the associated condenser coil 142 are connected to anassociated evaporator coil 130 by a refrigerant line 146. In a typicalembodiment, the variable-speed compressor 140 is, for example, asingle-stage compressor, a multi-stage compressor, a single-speedcompressor, or a variable-speed compressor. The variable-speedcirculation fan 110, sometimes referred to as a blower, is configured tooperate at different capacities (i.e., variable motor speeds) tocirculate air through the HVAC system 100, whereby the circulated air isconditioned and supplied to the enclosed space 101.

In various embodiments, as described in greater relative to PIG. 3, thevariable-speed compressor 140 may be representative of a compressorsystem including multiple compressors of the same or differentcapacities, one or more of which may be variable-speed compressors. Inthese embodiments, such compressors may include any appropriatearrangement of compressors (e.g., in series and/or in parallel). In someof these embodiments, such compressors may operate in tandem and sharedischarge lines and suction lines. In addition, or alternatively, suchcompressors may be independently operable in some implementations. Forexample, a first compressor may be allowed to operate and a secondcompressor may be restricted from operation. Compressor operations mayinclude full-load operations and part-load operations. A full-loadoperation may include operation of each compressor. A part-loadoperation may include allowing operation of one or more compressors andrestricting operation of one or more other compressors. In someimplementations, a part-load operation may include operation of amultistage compressor at one of the low settings (e.g., when acompressor includes a high setting and at least one low setting).

In embodiments in which the variable-speed compressor 140 isrepresentative of multiple compressors, the evaporator coil 130 mayinclude a plurality of evaporator circuits that are apportioned amongthe compressors according to a suitable circuiting arrangement, whereeach compressor operates off of the evaporator circuits apportionedthereto. For example, in various embodiments, the evaporator coil 130may implement a row-split or intertwined circuiting arrangement.Although the variable-speed compressor 140 can be representative ofmultiple compressors as described above, for simplicity of descriptionand illustration, the variable-speed compressor 140 will be illustratedand described singly.

Still referring to FIG. 1, the HVAC system 100 includes an HVACcontroller 150 that is configured to control operation of the variouscomponents of the HVAC system 100 such as, for example, thevariable-speed circulation fan 110, the re-heat coil 120, and thevariable-speed compressor 140 to regulate the environment of theenclosed space 101. In some embodiments, the HVAC system 100 can be azoned system. In such embodiments, the HVAC system 100 includes a zonecontroller 180, dampers 185, and a plurality of environment sensors 160.In a typical embodiment, the HVAC controller 150 cooperates with thezone controller 180 and the dampers 185 to regulate the environment ofthe enclosed space 101.

The HVAC controller 150 may be an integrated controller or a distributedcontroller that directs operation of the HVAC system 100. In a typicalembodiment, the HVAC controller 150 includes an interface to receive,for example, thermostat calls, temperature setpoints, blower controlsignals, environmental conditions, and operating mode status for variouszones of the HVAC system 100. For example, in a typical embodiment, theenvironmental conditions may include indoor temperature and relativehumidity of the enclosed space 101. In a typical embodiment, the HVACcontroller 150 also includes a processor and a memory to directoperation of the HVAC system 100 including, for example, a speed of thevariable-speed circulation fan 110.

Still referring to FIG. 1, in some embodiments, the plurality ofenvironment sensors 160 are associated with the HVAC controller 150 andalso optionally associated with a user interface 170. The plurality ofenvironment sensors 160 provide environmental information within a zoneor zones of the enclosed space 101 such as, for example, temperature andhumidity of the enclosed space 101 to the HVAC controller 150. Theplurality of environment sensors 160 may also send the environmentalinformation to a display of the user interface 170. In some embodiments,the user interface 170 provides additional functions such as, forexample, operational, diagnostic, status message display, and a visualinterface that allows at least one of an installer, a user, a supportentity, and a service provider to perform actions with respect to theHVAC system 100. In some embodiments, the user interface 170 is, forexample, a thermostat of the HVAC system 100. In other embodiments, theuser interface 170 is associated with at least one sensor of theplurality of environment sensors 160 to determine the environmentalcondition information and communicate that information to the user. Theuser interface 170 may also include a display, buttons, a microphone, aspeaker, or other components to communicate with the user. Additionally,the user interface 170 may include a processor and memory that isconfigured to receive user-determined parameters such as, for example, arelative humidity of the enclosed space 101, and calculate operationalparameters of the HVAC system 100 as disclosed herein.

In a typical embodiment, the HVAC system 100 is configured tocommunicate with a plurality of devices such as, for example, acommunication device 155, a monitoring device 156, and the like. In atypical embodiment, the monitoring device 156 is not pail of the HVACsystem. For example, the monitoring device 156 is a server or computerof a third party such as, for example, a manufacturer, a support entity,a service provider, and the like. In other embodiments, the monitoringdevice 156 is located at an office of, for example, the manufacturer,the support entity, the service provider, and the like.

In a typical embodiment, the communication device 155 is a non-HVACdevice having a primary function that is not associated with HVACsystems. For example, non-HVAC devices include mobile-computing devicesthat are configured to interact with the HVAC system 100 to monitor andmodify at least some of the operating parameters of the HVAC system 100.Mobile computing devices may be, for example, a personal computer (e.g.,desktop or laptop), a tablet computer, a mobile device (e.g., smartphone), and the like. In a typical embodiment, the communication device155 includes at least one processor, memory and a user interface, suchas a display. One skilled in the art will also understand that thecommunication device 155 disclosed herein includes other components thatare typically included in such devices including, for example, a powersupply, a communications interface, and the like.

The zone controller 180 is configured to manage movement of conditionedair to designated zones of the enclosed space 101. Each of thedesignated zones include at least one conditioning or demand unit suchas, for example, the evaporator coil 130 and the re-heat coil 120 and atleast one user interface 170 such as, for example, the thermostat. Thezone-controlled HVAC system 100 allows the user to independently controlthe temperature in the designated zones. In a typical embodiment, thezone controller 180 operates electronic dampers 185 to control air flowto the zones of the enclosed space 101.

In some embodiments, a data bus 190, which in the illustrated embodimentis a serial bus, couples various components of the HVAC system 100together such that data is communicated therebetween. In a typicalembodiment, the data bus 190 may include, for example, any combinationof hardware, software embedded in a computer readable medium, or encodedlogic incorporated in hardware or otherwise stored (e.g., firmware) tocouple components of the HVAC system 100 to each other. As an exampleand not by way of limitation, the data bus 190 may include anAccelerated Graphics Port (AGP) or other graphics bus, a Controller AreaNetwork (CAN) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT)interconnect, an INFINIBAND interconnect, a low-pin-count (LPC) bus, amemory bus, a Micro Channel Architecture (MCA) bus, a PeripheralComponent Interconnect (PCI) bus, a PCI-Express (PCI-X) bus, a serialadvanced technology attachment (SATA) bus, a Video Electronics StandardsAssociation local (VLB) bus, or any other suitable bus or a combinationof two or more of these. In various embodiments, the data bus 190 mayinclude any number, type, or configuration of data buses 190, whereappropriate. In particular embodiments, one or more data buses 190(which may each include an address bus and a data bus) may couple theHVAC controller 150 to other components of the HVAC system 100. In otherembodiments, connections between various components of the HVAC system100 are wired. For example, conventional cable and contacts may be usedto couple the HVAC controller 150 to the various components. In someembodiments, a wireless connection is employed to provide at least someof the connections between components of the HVAC system such as, forexample, a connection between the HVAC controller 150 and thevariable-speed circulation fan 110, the variable-speed compressor 140,or the plurality of environment sensors 160.

FIG. 2A is a schematic diagram of a package HVAC system 200 having are-heat loop 260. The package HVAC system 200 includes the refrigerantevaporator coil 130, the condenser coil 142, the compressor 140, and ametering device 202. In a typical embodiment, the metering device 202is, for example, a thermostatic expansion valve or a throttling valve.The refrigerant evaporator coil 130 is fluidly coupled to the compressor140 via a suction line 204. The compressor 140 is fluidly coupled to thecondenser coil 142 via a discharge line 206. The condenser coil 142 isfluidly coupled to the metering device 202 via a liquid line 208.

Still referring to FIG. 2A, during operation, low-pressure,low-temperature refrigerant is circulated through the refrigerantevaporator coil 130. The refrigerant is initially in a liquid/vaporstate. In a typical embodiment, the refrigerant is, for example, R-22,R-134a, R-410A, R-744, or any other suitable type of refrigerant asdictated by design requirements. Air from within the enclosed space 101,which is typically warmer than the refrigerant, is circulated around therefrigerant evaporator coil 130 by the circulation fan 110. In a typicalembodiment, the refrigerant begins to boil after absorbing heat from theair and changes state to a low-pressure, low-temperature, super-heatedvapor refrigerant. Saturated vapor, saturated liquid, and saturatedfluid refer to a thermodynamic state where a liquid and its vapor existin approximate equilibrium with each other. Super-heated fluid andsuper-heated vapor refer to a thermodynamic state where a vapor isheated above a saturation temperature of the vapor. Sub-cooled fluid andsub-cooled liquid refers to a thermodynamic state where a liquid iscooled below the saturation temperature of the liquid.

The low-pressure, low-temperature, super-heated vapor refrigerant isintroduced into the compressor 140 via the suction line 204. In atypical embodiment, the compressor 140 increases the pressure of thelow-pressure, low-temperature, super-heated vapor refrigerant and, byoperation of the ideal gas law, also increases the temperature of thelow-pressure, low-temperature, super-heated vapor refrigerant to form ahigh-pressure, high-temperature, superheated vapor refrigerant. Thehigh-pressure, high-temperature, superheated vapor refrigerant enters athree-way valve 262 where at least a portion of the high-pressure,high-temperature, superheated vapor refrigerant is diverted into are-heat feed line 264. The re-heat feed line 264 directs thehigh-pressure, high-temperature, superheated vapor refrigerant to are-heat coil 266. In certain embodiments, the re-heat coil 266 ispositioned adjacent to the evaporator coil 130. In some embodiments, there-heat coil 266 can be positioned in a supply duct 256 downwind fromthe evaporator coil 130. The re-heat coil 266 facilitates transfer of aportion of the heat stored in the high-pressure, high-temperature,superheated vapor refrigerant to air moving through the supply duct 256thereby heating the air in the supply duct 256. If the high-pressure,high-temperature, superheated vapor refrigerant is warmer, more heat canbe transferred to the air in the supply duct 256 thereby causing atemperature of the air in the supply duct 256 to be closer to atemperature of air in a return duct 254. After leaving the re-heat coil266, the high-pressure, high-temperature, superheated vapor refrigeranttravels through a re-heat return line 270 and enters the condenser coil142.

Outside air is circulated around the condenser coil 142 by avariable-speed condenser fan 210. The outside air is typically coolerthan the high-pressure, high-temperature, superheated vapor refrigerantpresent in the condenser coil 142. Thus, heat is transferred from thehigh-pressure, high-temperature, superheated vapor refrigerant to theoutside air. Removal of heat from the high-pressure, high-temperature,superheated vapor refrigerant causes the high-pressure,high-temperature, superheated vapor refrigerant to condense and changefrom a vapor state to a high-pressure, high-temperature, sub-cooledliquid state. The high-pressure, high-temperature, sub-cooled liquidrefrigerant leaves the condenser coil 142 via the liquid line 208 andenters the metering device 202.

In the metering device 202, the pressure of the high-pressure,high-temperature, sub-cooled liquid refrigerant is abruptly reduced. Invarious embodiments where the metering device 202 is, for example, athermostatic expansion valve, the metering device 202 reduces thepressure of the high-pressure, high-temperature, sub-cooled liquidrefrigerant by regulating an amount of refrigerant that travels to therefrigerant evaporator coil 130. Abrupt reduction of the pressure of thehigh-pressure, high-temperature, sub-cooled liquid refrigerant causessudden, rapid, evaporation of a portion of the high-pressure,high-temperature, sub-cooled liquid refrigerant, commonly known as flashevaporation. The flash evaporation lowers the temperature of theresulting liquid/vapor refrigerant mixture to a temperature lower than atemperature of the air in the enclosed space 101. The liquid/vaporrefrigerant mixture leaves the metering device 202 and returns to therefrigerant evaporator coil 130.

FIG. 2B is a schematic diagram of a split HVAC system 200′ having are-heat loop 280. The split HVAC system 200′ is similar in operation andconstruction to the package HVAC system 200. In the split HVAC system200′, the high-pressure, high-temperature, saturated liquid refrigerantleaves the condenser coil 142 via the liquid line 208 and enters athree-way valve 282 where a portion of the high-pressure,high-temperature, saturated liquid refrigerant is diverted into are-heat feed line 284. The re-heat feed line 284 directs thehigh-pressure, high-temperature, saturated liquid refrigerant to are-heat coil 286. In certain embodiments, the re-heat coil 286 ispositioned adjacent to the evaporator coil 130. In some embodiments, there-heat coil 286 can be positioned in the supply duct 256 downwind fromthe evaporator coil 130. The re-heat coil 286 facilitates transfer of aportion of the heat stored in the high-pressure, high-temperature,saturated liquid refrigerant to air moving through the supply duct 256thereby heating the air in the supply duct 256. If the high-pressure,high-temperature, saturated liquid refrigerant is warmer, more heat canbe transferred to the air in the supply duct 256 thereby causing atemperature of the air in the supply duct 256 to be closer to atemperature of air in the return duct 254. After leaving the re-heatcoil 286, the high-pressure, high-temperature, saturated liquidrefrigerant travels through a re-heat return line 290 and enters themetering device 202.

referring to FIGS. 2A-B collectively, a first temperature sensor 250 isdisposed in a return duct 254 and a second temperature sensor 252 isdisposed in a supply duct 256. In a typical embodiment, the firsttemperature sensor 250 and the second temperature sensor 252 are, forexample, thermistors; however, in other embodiments, the firsttemperature sensor 250 and the second temperature sensor 252 may bethermocouples, thermometers, or other appropriate devices as dictated bydesign requirements. The first temperature sensor 250 measures atemperature of air moving through the return duct 254 and the secondtemperature sensor 252 measures a temperature of air moving through thesupply duct 256.

The first temperature sensor 250 and the second temperature sensor 252transmit signals to the HVAC controller 150 corresponding to airtemperature values in the return duct 254 and the supply duct 256,respectively. The signals transmitted by the first temperature sensor250 and the second temperature sensor 252 are illustrated by arrows 257and 259, respectively. The first temperature sensor 250 and the secondtemperature sensor 252 may communicate with the HVAC controller 150 via,for example, a wired connection or a wireless connection.

An SST sensor 232 is disposed within or in relation to the evaporatorcoil 130. In various embodiments, the SST sensor 232 may be, forexample, a thermocouple, a thermometer, a pressure transducer, athermostat, a thermistor, or any other appropriate sensor for measuringSST. The SST sensor 232 measures SST and transmits the SST to the HVACcontroller 150. In some embodiments, the SST sensor 232. may be disposedon an exterior surface of the evaporator coil 130 thereby using anevaporator coil 130 surface temperature as a proxy measurement for theSST. Communication between the SST sensor 232 and the HVAC controller150 is illustrated by arrow 234. In a typical embodiment, the SST sensor232 continuously measures the SST; however, in other embodiments, theSST sensor 232 measures the SST at periodic time intervals such as, forexample, every five seconds. In a typical embodiment, the SST sensor 232is electrically coupled to the HVAC controller 150 via a wiredconnection however, in other embodiments, the SST sensor 232 isconnected to the HVAC controller 150 via a wireless connection. Incertain embodiments in which the variable-speed compressor 140 isrepresentative of multiple compressors, the SST sensor 232. can berepresentative of a plurality of such sensors. For example, a pluralityof sensors similar to the SST sensor 232 may be positioned for measuringSST at different locations at or on the evaporator coil 130.

In various embodiments, the HVAC controller 150 can use the SST sensor232 to modulate compressor speed, when deemed appropriate, to preventevaporator coil freeze. In various embodiments, the HVAC controller 150can monitor the SST sensor 232. When the SST is less than a minimum SSTthreshold, the HVAC controller can modulate the compressor speed inaccordance with a configurable algorithm. Examples of modulatingcompressor speed in response to changes in SST will be describedrelative to FIGS. 5-9.

FIG. 3 illustrates an example implementation 300 involving multiplecompressors. The implementation 300 includes compressors 340 a, 340 b,340 c and 340 d that are operable to operate off of an evaporator coil330 that includes multiple evaporator circuits 345. SST sensors 332 a,332 b, 332 c and 332 d (collectively, SST sensors 332) are disposed indifferent locations in or on the evaporator coil 330 and are operable tomeasure SST. In general, the SST sensors 332 can each operate asdescribed relative to the SST sensor 232 of FIGS. 2A-B. It should beappreciated that the number and arrangement of compressors, SST sensors,and evaporator circuits can be varied to suit a given implementation.For example, various implementations may include more or fewer than fourcompressors and/or more or fewer than four SST sensors.

FIG. 4 is a flow diagram of a process 400 for configurably modulatingcompressor speed during re-heat dehumidification mode to preventevaporator coil freeze. In various embodiments, the process 400 onlyexecutes when a given HVAC system, such as the HVAC system 100 of FIG.1, is operating in re-heat dehumidification mode. For illustrativepurposes, the process 400 will be described relative to FIGS. 1, 2A-Band 3.

At block 402, the HVAC controller 150 monitors the SST sensor 232. Asdescribed previously, particularly with reference to FIG. 3, the SSTsensor 232 can be representative of multiple SST sensors. In theseembodiments, the monitoring can include monitoring, for example,multiple SST sensors similar to the SST sensors 332 of FIG. 3. Asdescribed previously, each SST sensor 232 periodically transmits SSTs tothe HVAC controller 150.

At block 404, the HVAC controller 150 determines a reference SST(SST_(REF)) via each SST sensor 232. In general, SST_(REF) can be anyvalue deemed to represent SST for all or part of the evaporator coil130. In various embodiments, SST_(REF) can be a result of an automatedanalysis or computation using SSTs transmitted by each SST sensor 232.For example, block 404 can involve the HVAC controller 150 determining aplurality of SSTs (e.g., a most recent SST for each SST sensor 232) anddetermining a minimum SST of the plurality of SSTs, where the minimumSST serves as SST_(REF). In embodiments in which the SST sensor 232represents only a single SST sensor SST_(REF) can be the most recent SSTprovided by that single SST sensor.

At decision block 406, the HVAC controller 150 determines whetherSST_(REF) is below a configurable minimum SST threshold (SST_(Thresh)).In various embodiments, SST_(Thresh) can be set to a value that isconfigurably above an applicable freezing point. In various examples inwhich the applicable freezing point is 32° F., SST_(Thresh) may be setto 33° F., 34° F., 36° F., 38° F. or any other suitable temperature. Ifit is determined at decision block 406 that SST_(REF) is not belowSST_(Thresh), the process 400 proceeds to block 412.

At block 412, the HVAC controller 150 continues to use, or shifts to, apreviously established or standard compressor speed (CS_(STD)). CS_(STD)may correspond to a standard demand-based value that would otherwise beused, for example, in re-heat dehumidification mode in the absence ofthe process 400. In various cases, if, for example, the HVAC controller150 had previously been using a modified compressor speed, block 412 caninclude modulating a speed of the variable-speed compressor 140 incorrespondence to the standard demand-based value. From block 412, theprocess 400 returns to the block 402 and executes as describedpreviously.

If it is determined at the decision block 406 that SST_(REF) is belowSST_(Thresh), the process 400 proceeds to block 408. At block 408, theHVAC controller 150 determines a modified compressor speed (CS_(MOD))based on SST_(REF). In some embodiments, CS_(MOD) can vary withSST_(REF) between a minimum value (CS_(MIN)) and CS_(STD). In a typicalembodiment, if SST_(REF) is decreasing and a current compressor speed isgreater than CS_(MIN), block 408 will amount to decreasing thecompressor speed. Conversely, in a typical embodiment, if SST_(REF) isincreasing, although remaining below SST_(Thresh), block 408 will amountto increasing the compressor speed. In this fashion, the HVAC controller150 can determine a modified compressor speed in each iteration throughthe block 408 in response to a then-existing SST_(REF). At block 410,the HVAC controller 150 modulates a speed of the variable-speedcompressor 140 in correspondence to CS_(MOD). After block 410, theprocess 400 returns to block 402 and executes as described previously.In various embodiments, the process 400 can continue to execute untilterminated by a user or administrator or until other suitable stopcriteria is satisfied.

Equation 1 below provides an example of how CS_(MOD) may be determined,for example, during the block 408 of the process 400 of FIG. 4. In theexample of Equation 1, CS_(MOD) varies linearly with SST_(REF) betweenCS_(MIN) and CS_(STD). where SST_(LIM) represents a suitable value thatis configurably close to an applicable freezing point such as, forexample, 32° F. In certain embodiments, with reference to the process400 of FIG. 4, all compressor speeds may be expressed as percentages ofan applicable maximum speed. According to this example, CS_(STD) canequal any suitable percentage representative of re-heat compressordemand such as, for example, 60, 80, 100 or the like. Similarly,CS_(MIN) can represent any suitable percentage that is less thanCS_(STD) such as, for example, 30, 40, 50, 60 or the like.

$\begin{matrix}{{CS}_{MOD} = {{MAX}\left\lbrack {{CS}_{MIN},{{MIN}\left( {{CS}_{STD},{{CS}_{MIN} + \text{ }\frac{\left( {{CS}_{STD} - {CS}_{MIN}} \right)}{\left( {{SST}_{Thresh} - {SST}_{Lim}} \right)*\left( {{SST}_{REF} - {SST}_{LIM}} \right)}}} \right)}} \right\rbrack}} & {{Equation}1}\end{matrix}$

Although an example of a linear function is provided above forsimplicity of description, one skilled in the art will appreciate thatCS_(MOD) can also vary non-linearly with SST_(REF) between CS_(MIN) andCS_(STD). For example, CS_(MOD) can be established using a polynomialfunction. Non-linear variation, such as by way of a polynomial function,can be advantageous to provide more significant and responsive changedepending on a value of SST_(REF). According to this example, in caseswhere compressor speed is being algorithmically decreased as describedpreviously, relatively lower values of SST_(REF) (e.g., values close toan applicable freezing point) can result in a steeper decrease toCS_(MOD) than relatively higher values according to a polynomial curve.Similarly, in cases where compressor speed is being algorithmicallyincreased as described previously, relatively higher values of SST_(REF)(e.g., values closer to SST_(Thresh)) can result in a steeper increaseto CS_(MOD) than relatively lower values according to the polynomialcurve.

In some embodiments, a process for configurably modifying compressorspeed, such as the process 400 of FIG. 4, or portions thereof, may onlybe performed in particular modes of a given HVAC system. These modesgenerally have operational characteristics that lower SST and thusincrease a risk of evaporator coil freeze. For example, with referenceto the HVAC system 100 of FIG. 1, the process 400 may only be performedwhen the HVAC system 100 is in re-heat dehumidification mode. Inaddition, or alternatively, in certain embodiments, some portions of theprocess 400 may be performed in Al modes of the HVAC system 100 (e.g.,blocks 402-404), while functionality to modify compressor speed (e.g.,blocks 406-412) may only be performed in re-heat dehumidification modeof the HVAC system 100. Other variations and potential preconditionswill be apparent to one skilled in the art after a detailed review ofthe present disclosure.

FIG. 5 is a graph 500 that shows example variation in compressor speeds502 a, 502 b and 502 c (collectively, compressor speeds 502) for threedifferent CS_(STD) values of 60, 80 and 100 percent, respectively. Inthe example of FIG. 5, SST_(Thresh) set to approximately 36° F. andSST_(LIM) is set to approximately 32° F. When SST_(REF) is belowSST_(Thresh), the compressor speeds 502 each correspond to a CS_(MOD)according to Equation 1 above, thereby varying linearly with SST_(REF)between CS_(MIN) and a respective CS_(STD). Otherwise, when SST_(REF) isnot below SST_(Thresh), the compressor speeds 502 each correspond totheir respective CS_(STD).

FIG. 6 is a graph 600 that illustrates example compressor-speedmodulation for an example multi-compressor HVAC system. In the exampleof FIG. 6, compressor C1 is a variable-speed compressor similar to thevariable-speed compressor 140 of FIG. The graph 600 shows that a stage-2re-heat dehumidification mode is entered between 06:30 and 07:00. Arrows602, 604 and 606 indicate that, as SST2 falls below SST_(Thresh) (i.e.,36° F. In this example), the example multi-compressor HVAC systemremains in stage-2 re-heat dehumidification mode and a compressor speed608 of the compressor C1 is algorithmically decreased in the fashiondescribed above. SST2 thereafter stabilizes in response to the decreasein the compressor speed 608.

For purposes of this patent application, the term computer-readablestorage medium encompasses one or more tangible computer-readablestorage media possessing structures. As an example and not by way oflimitation, a computer-readable storage medium may include asemiconductor-based or other integrated circuit (IC) (such as, forexample, a field-programmable gate array (FPGA) or anapplication-specific IC (ASIC)), a hard disk, an HDD, a hybrid harddrive (MID), an optical disc, an optical disc drive (ODD), amagneto-optical disc, a magneto-optical drive, a floppy disk, a floppydisk drive (FDD), magnetic tape, a holographic storage medium, asolid-state drive (SSD), a RAM-drive, a SECURE DIGITAL card, a SECUREDIGITAL drive, a flash memory card, a flash memory drive, or any othersuitable tangible computer-readable storage medium or a combination oftwo or more of these, where appropriate.

Particular embodiments may include one or more computer-readable storagemedia implementing any suitable storage. In particular embodiments, acomputer-readable storage medium implements one or more portions of theHVAC controller 150, one or more portions of the user interface 170, oneor more portions of the zone controller 180, or a combination of these,where appropriate. In particular embodiments, a computer-readablestorage medium implements RAM or ROM. In particular embodiments, acomputer-readable storage medium implements volatile or persistentmemory. In particular embodiments, one or more computer-readable storagemedia embody encoded software.

In this patent application, reference to encoded software may encompassone or more applications, bytecode, one or more computer programs, oneor more executables, one or more instructions, logic, machine code, oneor more scripts, or source code, and vice versa, where appropriate, thathave been stored or encoded in a computer-readable storage medium. Inparticular embodiments, encoded software includes one or moreapplication programming interfaces (APIs) stored or encoded in acomputer-readable storage medium. Particular embodiments may use anysuitable encoded software written or otherwise expressed in any suitableprogramming language or combination of programming languages stored orencoded in any suitable type or number of computer-readable storagemedia. In particular embodiments, encoded software may be expressed assource code or object code. In particular embodiments, encoded softwareis expressed in a higher-level programming language, such as, forexample, C, Python, Java, or a suitable extension thereof. In particularembodiments, encoded software is expressed in a lower-level programminglanguage, such as assembly language (or machine code). In particularembodiments, encoded software is expressed in JAVA. In particularembodiments, encoded software is expressed in Hyper Text Markup Language(HTML), Extensible Markup Language (XML), or other suitable markuplanguage.

Depending on the embodiment, certain acts, events, or functions of anyof the algorithms described herein can be performed in a differentsequence, can be added, merged, or left out altogether (e.g., not alldescribed acts or events are necessary for the practice of thealgorithms). Moreover, in certain embodiments, acts or events can beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors or processor cores or onother parallel architectures, rather than sequentially. Although certaincomputer-implemented tasks are described as being performed by aparticular entity, other embodiments are possible in which these tasksare performed by a different entity.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or moreembodiments or that one or more embodiments necessarily include logicfor deciding, with or without author input or prompting, whether thesefeatures, elements and/or states are included or are to be performed inany particular embodiment.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the devices or algorithms illustrated can be madewithout departing from the spirit of the disclosure. As will berecognized, the processes described herein can be embodied within a formthat does not provide all of the features and benefits set forth herein,as some features can be used or practiced separately from others. Thescope of protection is defined by the appended claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A method of preventing evaporator coil freeze ina heating, ventilation and air conditioning (HVAC) system, the methodcomprising, by a controller in the HVAC system: determining a referencesaturated suction temperate (SST) via a sensor disposed in relation toan evaporator coil in the HVAC system, wherein the HVAC system isoperating in re-heat dehumidification mode; determining whether thereference SST is below a minimum SST threshold; responsive to adetermination that the reference SST is below the minimum SST threshold,determining a decreased compressor speed; and modulating avariable-speed compressor in the HVAC system in correspondence to thedecreased compressor speed.
 2. The method of claim 1, wherein thedecreased compressor speed varies linearly with the reference SSTbetween a first value and a second value.
 3. The method of claim 2,wherein the first value comprises a minimum value and the second valuecomprises a pre-established value based on re-heat compressor demand. 4.The method of claim 1, wherein the decreased compressor speed variesnon-linearly with the reference SST between a first value and a secondvalue.
 5. The method of claim 1, wherein the determining the referenceSST comprises: determining a plurality of SSTs via a plurality ofsensors disposed in relation to the evaporator coil; and determining aminimum SST of the plurality of SSTs, wherein the minimum SST is thereference SST.
 6. The method of claim 5, wherein the evaporator coilcomprises a plurality of evaporator circuits apportioned to a pluralityof compressors in the HVAC system, the plurality of sensors comprisingat least one sensor disposed in relation to each of the plurality ofevaporator circuits.
 7. The method of claim 1, comprising: determining asecond reference SST via the sensor; determining a modified compressorspeed; and modulating the variable-speed compressor in correspondence tothe modified compressor speed.
 8. The method of claim 1, comprising:determining a second reference SST via the sensor; determining whetherthe second reference SST is below the minimum SST threshold; andresponsive to a determination that the second reference SST is not belowthe minimum SST threshold, modulating the variable-speed compressor incorrespondence to a standard demand-based value for the re-heatdehumidification mode.
 9. The method of claim 1, comprising: determiningwhether the HVAC system is in re-heat dehumidification mode; and whereinthe determining whether the reference SST is below the minimum SSTthreshold is performed responsive to a determination that the HVACsystem is in re-heat dehumidification mode.
 10. The method of claim 1,comprising: determining whether the HVAC system is in re-heatdehumidification mode; and wherein the determining the reference SST isperformed responsive to a determination that the HVAC system is inre-heat dehumidification mode.
 11. A heating, ventilation, and airconditioning (HVAC) system comprising: an evaporator coil; a re-heatcoil; a condenser coil; a sensor disposed in relation to the evaporatorcoil; a compressor fluidly coupled to the condenser coil, the evaporatorcoil and the re-heat coil; and a controller operatively coupled to thecompressor, wherein the controller is operable to perform a methodcomprising: determining a reference saturated suction temperate (SST)via the sensor, wherein the HVAC system is operating in re-heatdehumidification mode; determining whether the reference SST is below aminimum SST threshold; and responsive to a determination that thereference SST is below the minimum SST threshold, determining adecreased compressor speed; and modulating a variable-speed compressorin the HVAC system in correspondence tot the decreased compressor speed.12. The HVAC system of claim 11, wherein the decreased compressor speedvaries linearly with the reference SST between a first value and asecond value.
 13. The HVAC system of claim 12, wherein the first valuecomprises a minimum, value and the second value comprises apre-established value based on re-heat compressor demand.
 14. The HVACsystem of claim 12, wherein the decreased compressor speed variesnon-linearly with the reference SST between a first value and a secondvalue.
 15. The HVAC system of claim 11, comprising: a plurality ofsensors disposed in relation to the evaporator coil; and wherein thedetermining the reference SST comprises: determining a plurality of SSTsvia a plurality of sensors disposed in relation to the evaporator coil;and determining a minimum SST of the plurality of SSTs, wherein theminimum SST is the reference SST.
 16. The HVAC system of claim 15,wherein the evaporator coil comprises a plurality of evaporator circuitsapportioned to a plurality of compressors in the HVAC system, theplurality of sensors comprising at least one sensor disposed in relationto each of the plurality of evaporator circuits.
 17. The HVAC system ofclaim 11, the method comprising: determining a second reference SST viathe sensor; determining a modified compressor speed; and modulating thevariable-speed compressor in correspondence to the modified compressorspeed.
 18. The HVAC system of claim 11, the method comprising:determining a second reference SST via the sensor; determining whetherthe second reference SST is below the minimum SST threshold; andresponsive to a determination that the second reference SST is not belowthe minimum SST threshold, modulating the variable-speed compressor incorrespondence to a standard demand-based value for the re-heatdehumidification mode.
 19. The HVAC system of claim 11, the methodcomprising: determining whether the HVAC system is in re-heatdehumidification mode; and wherein the determining whether the referenceSST is below the minimum SST threshold is performed responsive to adetermination that the HVAC system is in re-heat dehumidification mode.20. A computer-program product comprising a non-transitorycomputer-usable medium having computer-readable program code embodiedtherein, the computer-readable program code adapted to be executed toimplement a method of preventing evaporator coil freeze in a heating,ventilation and air conditioning (HVAC) system: determining a referencesaturated suction temperate (SST) via a sensor disposed in relation toan evaporator coil in the HVAC system, wherein the HVAC system isoperating in re-heat dehumidification mode; determining whether thereference SST is below a minimum SST threshold; responsive to adetermination that the reference SST is below the minimum SST threshold,determining a decreased compressor speed; and modulating avariable-speed compressor in the HVAC system in correspondence to thedecreased compressor speed.